Recently, internal combustion engines of vehicles have been equipped with closed loop air-fuel control system. These closed loop systems are employed for providing a desired air-fuel ratio of the air-fuel mixture fed to the engine so that harmful components contained in the exhaust gases are effectively reduced in a catalytic converter disposed in the exhaust gas passage of the engine. Further the desired air/fuel ratio contributes to the stability of the engine operation.
When a malfunction occurs in the closed loop system, it is preferable to disable the closed loop system in order to prevent an undesirable control with which a rich or lean air-fuel mixture may be fed to the engine. A technical expert of Nissan Motor Company of Japan has already proposed a display device which illuminatingly displays the frequency of a control signal obtained in a closed loop control system in Japanese Utility Model Application Ser. No. 50-140049. In the above-mentioned device a light emitting diode is connected via a resistor to the output of the comparator which is utilized for comparing the magnitude of signal of a gas sensor such as a zirconium oxygen sensor with a reference signal or to the output of the control unit which includes P-I (proportional-integral) controller connected to the comparator. The light emitting diode emits light, for instance if the device is connected to the output of the comparator, when the magnitude of the output signal of the gas sensor is greater than that of the reference signal. It is already known that the light emitting diode emits light in response to the fluctuation of the gas sensor output three or four times per a second when the crank shaft of the engine rotates at high speed, i.e., from 5000 rpm to 6000 rpm while the same emits light once in three to four seconds when the engine speed is low, i.e., for instance at an idling operation.
Above-mentioned device has some advantages viz. the construction thereof is simple and it requires little electric power. However, the display device includes disadvantage or a drawback which will be discussed hereinbelow. When the temperature of the gas sensor is low, the internal resistance of the gas sensor is so high that maximum and minimum values of the output signal of the sensor approach 0 volt so an inadequate difference between these maximum and minimum values occurs. Therefore, desirable closed loop control can not performed. In order to prevent this undesirable feedback control the closed loop control system is usually disabled when the temperature of the gas sensor is below a predetermined level. Since the light emitting diode of the display device does not emit light when the closed loop control system is disabled, the driver of the vehicle is apt to misunderstand that the closed loop control system is out of order or abnormal unless the vehicle driver is aware of the function of the closed loop control system in detail.
If the closed loop control system does not include means for disabling the system, frequency of the control signal produced by the closed loop control system is somewhat different from that during normal conditions when the temperature of the gas sensor is below a predetermined level. Therefore, the light emitting diode emits light with an irregular frequency whereby the vehicle driver may misunderstand that a malfunction has occured in the closed loop system.